Photoreceptor pigments which control various photobiological responses at the level of cellular metabolism have been sought by examining photoresponsive organisms for light-induced absorbance changes. In the mycelium of Neurospora crassa, where light exerts a control on the phase of circadian rhythms, also causes the photo-reduction of a b-type cytochrome with the same action spectrum that is found for the photocontrol of the circadian rhythm. We have obtained such light-induced absorbance changes in cell-free preparations of the mycelium. It is apparent that a flavin absorbs the light and mediates redox changes of the cytochrome. We have also studied similar reactions in artificial model systems containing flavins, cytochromes, electron donors and various inhibitors to define these photoreactions further.